1. Field of the Invention
The present invention relates generally to the field of lithography. More specifically, the present invention is directed to a container for holding a mask for a lithographic apparatus, and a mask transport system configured to transport a mask into and out of a lithographic apparatus.
2. Description of Related Art
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device or patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time.
In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machines. In one type of lithographic projection apparatus, each target portion is irradiated by projecting the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step and repeat apparatus. In an alternative apparatus—commonly referred to as a step and scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), and the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion implantation (doping), metallization, oxidation, chemo mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0 07 067250 4, incorporated herein by reference.
For the sake of simplicity, the projection system of a lithographic projection apparatus may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. Its radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both of which are incorporated herein by reference.
The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table, or mask support, ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Conventionally, the mask table has been positioned such that radiation is passed from the illumination system through the mask, the projection system, and onto the substrate. Such masks are known as transmissive masks since they selectively allow the radiation from the illumination system to pass through, thereby forming a pattern on the substrate. Such masks must be supported so as to allow the transmission of light therethrough. This has conventionally been achieved by using a vacuum in the table at a perimeter zone of the mask so that the atmospheric air pressure clamps the mask to the table.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and, hence, higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light of 365 nm, 248 nm, and 193 nm generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV, with a wavelength in a range of about 5-20 nm) radiation, and possible sources include laser-produced plasma sources, discharge sources, or synchrotron radiation sources, examples of which are, for example, disclosed in European patent applications EP 1,109,427 A and EP 1,170,982 A, both of which are incorporated herein by reference.
Since no materials are known to date to be sufficiently transparent to EUV radiation, a lithographic projection apparatus employing EUV radiation is envisaged to employ a reflective mask having a multilayer coating of alternating layers of different materials, for example, in the order of 50 periods of alternating layers of molybdenum and silicon or other materials, such as, for example, disclosed in European patent application EP 1,065,532 A, incorporated herein by reference. The size of the features to be imaged in EUV lithography makes the imaging process very sensitive to any contamination present on the mask. It is foreseen that any contaminant particles having a dimension from approximately 25 nm will result in defects present in devices fabricated in the substrate. Therefore, masks are often provided with a pellicle, as will be known to a person skilled in the art. A pellicle makes a mask less sensitive to contamination, since the contamination particles will fall on the pellicle instead of the mask and, as a result of that, will be out of focus.
Pellicles cannot be employed for EUV radiation since they will not be sufficiently transparent to EUV radiation. Particle contamination on the pattern-bearing reflective surface of the mask would therefore lead to defective devices fabricated and must be prevented.
Moreover, the reflective mask is envisaged to be held at its backside on the mask table by electrostatic forces on a mask-bearing surface to be able to meet the very stringent requirements for EUV mask positioning. Any contaminant particle present in between the backside of the mask and the mask-bearing surface of the mask table will cause the mask to be oriented at a tilt with respect to the proper orientation. Since the projection system will be non-telecentric on the object side, because a reflective mask is used (more information on this problem can be derived from European patent application EP 1,139,176 A, incorporated herein by reference), a tilt in the surface figure of the reflective mask surface will translate into a local shift of the pattern imaged onto the substrate. As a result, the imaged layer may not line up with earlier layers that have been processed in the substrates again leading to defective devices fabricated. Therefore, particle contamination on the backside surface of the mask should be prevented.
Molecular type of contamination, such as hydrocarbons and water, should also be prevented. Such contamination will have a detrimental effect on any of the optical components in the lithographic apparatus, including the mask. In all handling procedures of masks and substrates, care should be taken that their surfaces will remain clean from such molecular contamination. Masks and substrates may be stored and transported between various types of apparatus employing a storage box in which a protective environment is maintained, such as an environment that is evacuated or filled with an inert gas. The inside walls of such a storage box should also remain clean. However, while transferring a mask or substrate out of such a storage box to a device or apparatus for processing or employing such a substrate or mask, contamination, both particulate contamination and molecular contamination may be introduced onto mask or substrate or internal walls of a protective environment. One may transfer a mask or substrate through some intermediate chamber, for example, a load lock chamber, to a final environment for processing or use, but then very long pump down times may be required when the protective environment is to be evacuated.
According to the above, it will be understood that lithographic techniques are known to be very sensitive to contamination. Even very small contamination particles on the mask could be projected on the wafer or will cause an error in the positioning of the mask, as discussed above. This can disturb the resulting wafer in such a way, that the wafer becomes useless. Today, such production faults on the wafer can not be detected during the production process, but only afterwards. This involves the risk that a sequence of successively produced wafers are useless and need to be destroyed. Thus, it is important to keep the mask clean during the whole mask handling process and to scan the mask for contamination regularly. However, detecting contamination particles is a difficult and time consuming process. Especially contamination particles that are smaller than the dimensions of the relief on the mask surface are difficult to detect.
In order to minimize the risk of contamination, the mask is produced under very clean circumstances. After production, the mask is placed in a storage box, for storage and transportation to the lithographic projection apparatus. Also, when the mask is removed from the lithographic projection apparatus, for example for a scanning process, the mask is placed in the storage box. Storages boxes and methods for transferring the mask in and out of the lithographic projection apparatus in such a way that the storage box is only opened under very clean conditions are known. The storage box is adapted to be used in combination with a load lock.
In order to scan the mask for contamination, the mask needs to be removed from the lithographic projection apparatus and transported to a scanning assembly. Such a scanning assembly will be explained below, with reference to FIGS. 2a and 2b. Transportation from the lithographic projection apparatus to the scanning assembly and vice versa, can be done in the transport box. However, in order to scan the mask, it is necessary to get the mask out of the storage box. Scanning of the mask can not be done while the mask is inside the storage box. Opening of the storage box forms a big risk for particle contamination. This means that inspection for contamination is a contaminating action. Also, after inspection, the mask is placed in the storage box again, causing possible particle contamination. The result of the scanning process can therefore never be guaranteed.
Second, when kinematic frames need to be applied to the mask after being written, it must be done by also getting the mask out of its storage box and exposing it to a clean room environment. This is also a risk of particle contamination.